Proportional counters are one of the typical types of devices used for determining the presence of ionizing radiations and to determine the quantity and energy thereof. Such devices utilize an anode typically in the form of one or more wires maintained at a selected potential, and one or more cathodes at some lesser potential. Alternatively, the anode can be at ground potential, and the cathode(s) at a negative potential, or the cathode(s) at ground potential and the anode at some positive potential. The counters are filled with a gas of a type wherein interaction with the ionizing radiation produces charged particles that move to the anode and cathode(s).
A special form of proportional counters is a position sensitive proportional counter whereby the position within the counter of the interaction of the radiation (e.g., x-ray) with the gas can be determined. This position information is obtained by analyzing the signal produced on the anode or the cathode as a function of the point closest to that at which the interaction occurs. In order to obtain this signal, the anode (or the cathode if that is to be used), must have a given resistance. Typically, the anode wires are carbon-coated wires. An alternative anode wire is very fine stainless steel. In one type of position sensitive proportional counter, a cathode thereof is made of a serpentine-type resistance wire supported upon a substrate, with opposite ends thereof connected to circuitry to determine the point of initiation of a signal along that serpentine wire.
These position sensitive detectors of the prior art present certain problems in their construction and use. For example, the carbon-coated anode wires have a limited lifetime due to their contamination by the radiation-induced decomposition of the counter filling gas. This problem also affects very fine metallic anode wires. With carbon-coated fibers this changes the resistance, usually non-uniformly. For metal wires, the resistance does not change but the wire diameter increases resulting in a decrease in the gain. These changes result in degredation of both the spatial and the energy resolution. Furthermore, anode wires are only very difficulty shaped to produce a curved position sensitive proportional counter.
Certain of the prior art position sensitive proportional counters have utilized a cathode formed by a resistance wire "meandering" across a support substrate in a generally serpentine path. These are typically 25 .mu.m wires hung on various spring, glass, or other supporting devices. For many applications, the spacing between adjacent wires is 1-2 mm and such construction is relatively easily accomplished. However, the wires or meanders are each parallel to each other. This spacing is much too great, however, to achieve high linear position resolution (e.g., 50 .mu.m) needed for crystallographic and diffraction research with x-rays. For this resolution a spacing of 100-150 .mu.m is optimum. However, when the spacing of wires is reduced to, for example, about 250 .mu.m, electrostatic interactions between adjacent wires cause serious problems of wire displacement and thus erode spacial resolution of the counter.
Accordingly, it is one object of the present invention to provide a high speed position sensitive detector wherein multiple metallic anode wires of relatively large diameter are arranged so as to distribute space charge within the detector over a wide area thus leading to both a longer detector lifetime and a higher count rate capability.
It is also an object to provide a high speed position sensitive proportional counter having a cathode meander wherein adjacent portions thereof are spaced at 100-150 .mu.m.
It is another object of the invention to provide a method for producing a cathode for a high speed position sensitive proportional counter, the cathode having a meander wherein adjacent portions thereof are spaced at 100-150 .mu.m.
It is another object of the present invention to provide a method by which the wires of a cathode "meander" are not parallel to each other, but are formed on the spokes of a wheel in order to make possible the detection of x-rays from a point source without parallax.
It is a further object of the present invention to provide a high speed curved position sensitive proportional detector for use in x-ray diffraction instruments, with a spacial resolution of at least 50 .mu.m.
These and other objects of the present invention will become more apparent upon a consideration of the following figures and a detailed description thereof.